Traditionally, thin film devices have been formed through processes such as photolithography. In a photolithographic process, a substrate is provided and at least one material layer is uniformly deposited upon the substrate. A photo-resist layer, also commonly known simply as photoresist, or even resist, is deposited upon the material layer, typically by a spin coating machine. A mask is then placed over the photoresist and light, typically ultra-violet (UV) light, is applied through the mask to expose portions of the photoresist. During the process of exposure, the photoresist undergoes a chemical reaction. Generally, the photoresist will react in one of two ways.
With a positive photoresist, UV light changes the chemical structure of the photoresist so that it is soluble in a developer. What “shows” therefore goes, and the mask provides an exact copy of the patterns which are to remain. What remains, can be, for example, the trace lines of a circuit. Photolithography may also be considered a 2D process, in that each layer of material is deposited and then masked. Although 3D structure may be created by stacking layers patterned via the 2D process, there is no inherent alignment feature between the layers.
A negative photoresist behaves in the opposite manner. That is, the UV exposure causes it to polymerize and not dissolve in the presence of a developer. As such, the mask is a photographic negative of the pattern to be left. Following the developing with either a negative or positive photoresist, blocks of photoresist remain. These blocks may be used to protect portions of the original material layer, or serve as isolators or other components.
Photolithography is a precise process applied to small substrates. In part, this small-scale application is due to the high cost of the photo masks. For the fabrication of larger devices, typically, rather than employing a larger and even more costly photo mask, a smaller mask is repeatedly used—a process that requires precise alignment.
As a photolithographic process typically involves multiple applications of materials, repeated masking and etching, issues of alignment between the thin film layers is of high importance. A photolithographic process is not well suited for formation of thin film devices on flexible substrates, where expansion, contraction or compression of the substrate may result in significant misalignment between material layers, thereby leading to inoperable thin film devices. In addition, a flexible substrate is not flat—it is difficult to hold flat during the imprinting process and thickness and surface roughness typically cannot be controlled as well as they can with glass or other non-flexible substrates.
The issue of flatness in photolithography can be a problem because the minimum feature size that can be produced by a given imaging system is proportional to the wavelength of the illumination divided by the numerical aperture of the imaging system. However, the depth of field of the imaging system is proportional to the wavelength of the illumination divided by the square of the numerical aperture. Therefore, as resolution is increased, the flatness of the substrate quickly becomes the critical issue.
It is desirable to have a method and system for formation of thin film devices over a flexible substrate. It is desirable that there not be limitations on the types of material used to form the thin film devices. The method and system should be simple, cost effective and capable of being easily adapted to existing technology.